With each day, it seems we learn more about the shadowy world of political advertising on digital platforms like Facebook, Google, and Twitter. As multiple investigations into the 2016 presidential election already indicate, it’s a world urgently in need of transparency.

In the US Senate, some lawmakers are rightly pushing for new legislation that would require social media companies to publicly disclose who’s buying online political ads, the cost of those ads, and who those ads are targeting. It’s a shame we don’t have such a federal law already, but federal regulation isn’t the only available answer to this problem.

In Seattle, under a municipal law that dates to 1977, we already regulate political advertising on digital platforms. That’s not because the writers of this law predicted the advent of the Internet. It’s because they defined political advertising as any advertising that uses a “means of mass communication” in order to appeal “directly or indirectly” for votes, financial support, or “other support” during any local election campaign.

Whether it came about from farsighted word choice or just plain luck, our legal language clearly applies to local political ads that are purchased on digital platforms with the aim of influencing Seattle voters. And while Mark Zuckerberg, the founder of Facebook, was still seven years away from being born when this bit of municipal code was laid down, our law’s language now offers some straightforward answers to the novel challenges his amazing mass communication machine has created for our democracy.

For example, Seattle’s municipal code requires commercial advertising businesses like Mr. Zuckerberg’s to maintain, in a manner that’s “open for public inspection,” the names and addresses of people purchasing political ads that target Seattle elections. It also requires the company to disclose “the exact nature and extent of advertising services rendered” (which, in the world of social media, would include ad targeting information). In addition, the law says the public must be allowed to inspect “books of account” showing the amount and manner of payment for these ad services.

Seattle’s law also contains an outright ban on “concealment” of the identity of any political ad purchaser and it says that a failure to properly disclose the money trail behind political ads can be met with up to a $5,000 fine for each violation.

In other words, if a journalist, an election watchdog, or anyone else in Seattle wants to follow the trail of digital media ad money as it relates to our local elections, they can walk right into the Seattle offices of Facebook, Google, or Twitter (“during normal business hours,” as the law says) and ask for an extensive accounting — just as interested parties already do when tracking political ads purchased on our local television stations.

Americans don’t have to wait for Congress to act on this issue, nor do we need to rely on the hope that digital platforms will police themselves.

The existence of this law also makes clear that Americans don’t have to wait for Congress to act on this issue, nor do we need to rely on the hope that digital platforms will police themselves. Nothing is stopping other cities and states from following our lead — either by searching their own election codes for laws that apply to digital publishers, or by creating new laws.

Imagine a spreading constellation of local demonstration projects taking on the under-regulated world of online political advertising. It would be great to see. To be sure, this isn’t the only type of transparency we need to prevent the growing list of online actions that attempted to exert influence on our 2016 presidential election. Sensible, urgent action from lawmakers in Washington, DC remains essential. But if Seattle’s municipal government can force Facebook, Google, and Twitter to open their political ad books — and if other cities, counties, and states can do the same — then the message will be clear: the federal government should, too.